Over recent months, the HS2 project has had opportunities for a number of celebrations as drives on many of the large-diameter rail tunnels were completed. Those TBM drives took place outside of central London, where more tunnelling is yet to be done.
At the edge of London, in the Chilterns and up near Birmingham, many different tunnel drives were required, and completed, for the twin-tubes of the high speed rail project. Those TBMs on Phase 1 that are finished have been dismantled.
It is rare for so many large TBMs to be active in the UK, and even moreso outside of London, and also for them to be completing their bores within relatively tight windows.
The TBM drives so far and their finishes are briefly described below. More are to come, in London, with two TBMs on the Euston Tunnel. Works for the latter were enabled by construction of the Atlas Road Logistics Tunnel, which was completed early 2024. The project also includes some cut-and-cover ‘green’ tunnels.
EDGE OF LONDON
HS2’s 13.5km-long Northolt Tunnel was nearing completion of boring by mid-year when the fourth, and last, TBM on this section of the project would arrive and finish its journey at the Green Park Way site, in Greenford, west London.
The Green Park Way site sits along Northolt Tunnel, resulting in the tunnelling works on either side. To the west side is a for the twin-tunnels to be arranged in two groups: to the west is the 8km-long, 8.80m-i.d. western section from West Ruislip; and, to the east is the 5.5km-long, 8.10m-i.d. section that runs closer to London, to North Acton.
Northolt Tunnel is one of five deep twin-bore tunnels on HS2’s 225km-long route. It is the second longest tunnel on the project and runs up to 35m below ground at its deepest point. This section of HS2 extends from West Ruislip via North Acton to new Old Oak Common super-hub station.
Contractor is Skanska Costain Strabag JV (SCS JV). It is also building the Euston Tunnel, in central London.
Coming in from West Ruislip and North Action, pairs of TBMs have been closing on the retrieval shafts at Green Park Way from opposite sides. The shafts were tough to build, in difficult ground with high groundwater, and on a confined site. All four Herrenknecht shields are being received at the site
A special engineering solution of a ‘steel can’ was developed to sit in the shafts and received the TBM, without dewatering the ground outside. The TBMs maintain pressure, advance and behind them the tunnel lining is completed, up to the shaft. With boring finished and tunnel sealed, groundwater is no longer a problem, enabling the ‘can’ to be depressurised and the shield withdrawn.
The first two machines to arrive, from West Ruislip, were lifted out earlier this year. They were 9.84m-diameter ‘Sushila’ (launched October 2022, finished Dec 2024) and ‘Caroline’ (finished April 2025).
The last two shields to complete are on the North Acton drives, and are 9.11m-diameter ‘Emily’ (completed her drive in June) and ‘Anne’ (soon to finish). They were launched early last year.
CHILTERNS
The 16.1km-long Chiltern Tunnel is the longest underground section of the HS2 line and tunnelling was undertaken by two 10.26m-diameter TBMs – ‘Florence’ and ‘Cecilia’ – boring the twin tubes.
The contractor is Align JV, which comprises Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick. The operate as an ‘integrated project team’ (IPT).
‘Florence’ broke through at the north portal in February 2024 followed by ‘Cecilia’ weeks later, in late March. They were variable density Herrenknecht machines, launched in mid-2021.
Innovations outside the machines saw D-walls for four of the tunnel’s five shafts being designed and built only as permanent works. Elimination of a two-step approach, with first temporary works, radically reduced the shaft diameters from 31m to 17m.
The project also includes long concrete shell extensions to the driven tunnels to run beyond the bored portals. With ventilation portholes along one side, they are designed to help manage airflow and minimise, or eliminate, the possibility of ‘sonic boom’ shocks created by the high-speed trains. Visually, they are interesting structures in their own right.
NEAR BIRMINGHAM
In July 2023, JV contractor Balfour Beatty Vinci (BBV) launched the 8.62m-diameter TBM ‘Mary Ann’ to bore of one of the twin tubes of the 5.6km-long Bromford Tunnel, which is a vital section of the project to take the HS2 line in and out of Birmingham.
‘Mary Ann’ completed her drive in May this year, reaching the Washwood Heath site which is to be the operational heart of HS2 in future.
The site will host HS2’s Depot, for serving and storing rolling stock, and the Network Integrated Control Centre to operate the high-speed railway in real-time.
The second, parallel tunnel is being bored by TBM ‘Elizabeth’ and completion is expected later this year.
